Field of the Invention and Related Art Statement
The present invention relates to an apparatus for servo-controlling an objective lens provided in a recording and/or reproducing apparatus using an optical record medium such as an optical card and an optical disk.
In a recording and/or reproducing apparatus using an optical card, a servo-control signal representing a relative deviation of an objective lens and a record medium is detected and the objective lens is moved relative to the record medium in accordance with the servo-control signal. For instance, the objective lens is moved in a focusing direction parallel to an optical axis of the objective lens in accordance with a focusing error signal and at the same time the objective lens is moved in a tracking direction perpendicular both to the optical axis of the objective lens and to a track direction in which a number of tracks extend in accordance with a tracking error signal. There have been proposed various servo-control apparatuses for use in the recording and/or reproducing apparatus using an optical card. For instance, in Japanese Patent Application Publication Kokai-sho No. 62-275325 there is disclosed a servo-control apparatus in which the servo-control signal is derived by detecting a difference in the intensity between light beams reflected from two guide tracks for defining a data track therebetween. In Japanese Patent Application Publications Kokai-sho Nos. 62-279523 and 63-7533, there is also described a servo-control apparatus in which a servo-control signal is derived by detecting light reflected by a single clock pattern track provided in each track unit Including a plurality of data tracks.
In the above mentioned known servo-control apparatuses, when the guide track and clock pattern track include defects such as dust particles and damaged portions, it becomes impossible to detect the servo-control signal such as the focusing error signal and tracking error signal, so that the servo-control could not be performed correctly. For instance, the objective lens is moved into a far-defocused position and the light spot is deviated from the data track, and therefore the data can no longer be correctly recorded and/or reproduced.